When was the first double stack magazine created?
Double stack magazines were well known in rifles in Browning's day. Not sure of the very first, but the 1893 Mausers were common, and used a double stack magazine.
The 1896 Broomhandle also used a double stack magazine. It was more like the rifle magazines than the pistol magazines we know today. It was fixed to the receiver, and fed from the top, even using strippers like a rifle.
The Savage .32 used a double stack magazine. There was another .32, a foreign one, whose name is all I can remember, it was called "Ne Plus Ultra".
Do you think it would have been incorporated into the 1911 design?
It would have been had the army requested it. JMB was very good about incorporating requested features into his designs. As already mentioned, the army would not have wanted a double stack magazine .45 pistol at this time.
To recap, JMB could have designed the M1911 as a double stack magazine pistol, if he had been requested to do so. There were previous designs to show that the concept would work. Even today, many people can not handle a double stack .45, so the army wisely didn't request one.